In recent years, a demand for wireless networks has been increasing with the development of mobile communication technologies. Particularly, active research efforts for integrating a local wireless network based on the Institute of Electrical and Electronics Engineers (IEEE) 802.11 standard and a mobile communication network such as a Code Division Multiple Access (CDMA) network and a Universal Mobile Telecommunications System (UMTS) network are being made. The mobile communication network advantageously supports wide coverage and mobility, but enables only low speed transmission with a maximum of two megabits per second (Mbps). Compared to the mobile communication network (e.g., the CDMA/UMTS), the IEEE 802.11 local wireless network has a narrower service coverage, but enables only high-speed data transmission from eleven Mbps to a maximum of 54 Mbps. So, the two communication networks are efficiently integrated and provide wide coverage and high-speed data transmission for a seamless service to users.
However, when an IEEE 802.11 based wireless Mobile Station (MS) does handover, most of handover delays occur in a channel scanning step of searching a target Access Point (AP).
In the conventional channel scanning method, there are a passive scan scheme and an active scan scheme. The passive scan scheme is a scheme in which a wireless MS accesses all possible channels and waits for a beacon signal that is periodically transmitted through a corresponding channel from an AP. The active scan scheme is a scheme in which, after accessing each channel and transmitting a probe request frame, the wireless MS receives a probe response frame from an AP.
In the passive scan scheme, because the wireless MS accesses each channel and waits until a beacon signal is received from an AP, a separate control signal is not necessary, so an overhead is not generated. However, there is a disadvantage such that, when the wireless MS may not be synchronized when the beacon signal is transmitted from the AP, a considerable channel scanning delay is induced.
When the wireless MS cannot acquire previous information on an adjacent AP, the wireless MS should wait for a maximum of 100 milliseconds (ms) so as to receive a beacon signal from the AP for each channel. Because the wireless MS has to scan all channels supported in the IEEE 802.11 standard, when assuming that a local wireless network supports n channels, a delay time of ‘n×beacon period’ is needed. This considerable delay time may lead to a considerable degradation of Quality of Service (QoS) caused by delay time and packet loss, in an MS receiving a real-time data service.
In contrast, the active scan scheme is a technique in which, unlike the passive scan scheme, after accessing each channel, a wireless MS sends probe request messages and, during a constant wait time, receives probe response messages from all APs that are in operation in a corresponding channel. Because the wait time for the probe response message of the wireless MS is substantially shorter than the beacon period, the active scan scheme has an advantage that the delay time is significantly reduced. However, because the wireless MS directly broadcasts a probe request message for AP search, this induces an additional overhead. Also, there is a disadvantage in that channel traffic load increases if a plurality of wireless MSs send probe request messages.
Accordingly, there is a need for a method and apparatus for reducing a delay time resulting from channel scanning in a wireless communication system.